In the past, there has been provided a power feed control device A as shown in FIG. 3A. For example, the power feed control device A (for example, refer to Japanese Patent Application Laid-Open No. 2009-234392) includes a plug 20 being configured to be attached to an external power supply such as a commercial AC power supply of 100V, a load connector 21 being configured to be attached to a load such as a vehicle, and a feed line. The feed line includes a first conductive wire L1 corresponding to a live wire and a second conductive wire L2 corresponding to a neutral wire. The power feed control device A is used, for example, for supplying an on-vehicle charger with electric power. The on-vehicle charger is configured, for example, to charge up a battery which a plug-in hybrid vehicle or an electric vehicle is equipped with.
For details, the power feed control device A includes the first conductive wire L1 and the second conductive wire L2 for supplying power from a power supply to a load, and a third conductive wire L3 corresponding to a ground wire. The third conductive wire L3 is connected with a ground terminal of the plug 20. A relay (switching device) 10, including contact points rp1, rp2 and contact open/close parts rs1, rs2, is inserted in the first and second conductive wires L1, L2. In addition, the power feed control device A includes a fourth conductive wire L4 and a control circuit 12. The fourth conductive wire L4 is provided separately from the conductive wires L1 to L3. The fourth conductive wire L4 is configured to transmit a signal to and from the load. The control circuit 12 is configured to be connected with the load through the fourth conductive wire L4. The control circuit 12 is configured to control to turn on and off the relay 10 according to a control signal supplied from the load through the fourth conductive wire L4. In addition, the power feed control device A includes a zero-phase-sequence current transformer ZCT and a leakage detection circuit 13. The zero-phase-sequence current transformer ZCT is arranged between the relay 10 and the plug 20, and is passed through by each of the first and second conductive wires L1, L2. The leakage detection circuit 13 is configured to detect occurrence of electrical leakage by detecting an unbalanced current flowing in the feed line (the first conductive wire L1 and the second conductive wire L2) via the zero-phase-sequence current transformer ZCT. If unbalance occurs in currents flowing in the feed line (the first conductive wire L1 and the second conductive wire L2), the zero-phase-sequence current transformer ZCT generates an induced current according to the unbalanced current. The leakage detection circuit 13 is configured to detect occurrence of electrical leakage based on the induced current. That is, the leakage detection circuit 13 is configured to detect occurrence of electrical leakage based on the unbalanced current flowing through the zero-phase-sequence current transformer ZCT. The control circuit 12 is configured to control to turn off the relay 10 when the occurrence of electrical leakage is detected through the leakage detection circuit 13.
In addition, this conventional example includes a voltage detection circuit 14 and a branch wire L5, and is configured to be able to detect occurrence of a contact welding in the relay 10. The voltage detection circuit 14 is connected with each of the first and second conductive wires L1, L2 between the relay 10 and the load connector 21. The voltage detection circuit 14 is configured to detect voltage of each of the first and second conductive wires L1, L2 between the relay 10 and the load connector 21. The branch wire L5 is a conductive wire, and branches from the voltage detection circuit 14. One end of the branch wire L5 is configured to be grounded (for example, connected with the third conductive wire L3). As shown in FIG. 3B, the voltage detection circuit 14 is configured to detect whether or not voltage is applied to the first conductive wire L1 and the second conductive wire L2, respectively. That is, the comparator CP1 of the voltage detection circuit 14 compares a threshold voltage Vth with voltage of the first conductive wire L1 (or the second conductive wire L2) after being smoothed by a condenser C1 and a resistor R1 and being peak held by a diode D1, a condenser C2, a zener diode Z1 and a resistor R2. In this conventional example, when the control circuit 12 controls to turn off the relay 10, if a contact welding is not occurred in the relay 10, the contact open/close parts rs1, rs2 separates from the contact points rp1, rp2, respectively. Then, each of the first and second conductive wires L1, L2 is broken. In this case, current does not flow into the branch wire L5. And the voltage detection circuit 14 does not detect voltage. On the other hand, if one of the contact points rp1, rp2 and one of the contact open/close parts rs1, rs2 are welded, even if the control circuit 12 tries to control to turn off the relay 10, one of the contact open/close parts rs1, rs2 cannot separate from the corresponding contact points rp1, rp2. In this case, current flows from the power supply to ground through the welded contact open/close part rs1 or rs2 and though the branch wire L5. Then, the voltage detection circuit 14 detects voltage. If detecting the above contradiction, that is, if voltage is detected through the voltage detection circuit 14 when the relay 10 is controlled to turn off, the control circuit 12 decides that a contact welding occurs in the relay 10.
This conventional example also includes a power supply circuit (not shown in figure). The power supply circuit is connected with each of the first and second conductive wires L1, L2 between the relay 10 and the plug 20, and connected with the third conductive wire L3. The power supply circuit is configured to supply the control circuit 12, the leakage detection circuit 13 and the voltage detection circuit 14 with electric power.
As described above, in this conventional example, the branch wire L5 branches from the voltage detection circuit 14, and one end of the branch wire L5 is configured to be grounded. Then, this conventional example is configured to be able to detect a welding of the contact points rp1 and rp2, by detecting the current flowing to the branch wire L5. For this reason, this conventional example can prevent troubles such as electric shock.
In this conventional example, when a current flows in the first and second conductive wires L1, L2, a current always flows in the branch wire L5, too. That is, a part of the current flowing in the first or second conductive wire L1, L2 from a power supply side to a load side is to flow into the branch wire L5. In addition, a part of the current flowing in the first or second conductive wire L1, L2 from a load side to a power supply side is to flow into the branch wire L5. Considering the magnitude of electric current flowing in the first and second conductive wires L1, L2 where the zero-phase-sequence current transformer ZCT is arranged, the magnitude of the current I1 flowing from the power supply to the load is always larger than the magnitude of the current I2 flowing from the load to the power supply by the magnitude of the current I3 flowing in the branch wire L5 toward ground. That is, the magnitude of the current flowing in the first conductive wire L1 is different from the magnitude of the current flowing in the second conductive wire L2 by the magnitude of the current flowing in the branch wire L5. Therefore, a small unbalanced current always flows through the zero-phase-sequence current transformer ZCT. Here, it should be noted that each of the current flowing in the first and second conductive wires L1, L2 is alternating current. Therefore, the direction of the current flowing in the first conductive wire L1 and the direction of the current flowing in the second conductive wire L2 alternately vary with time. In addition, the relation, which magnitude of the current is larger, flowing in the first conductive wire L1 or flowing in the second conductive wire L2, varies with time. However, the magnitude of the current I1 flowing from the power supply to the load is always larger than the magnitude of the current I2 flowing from the load to the power supply.
In this conventional example, in order to prevent the leakage detection circuit 13 from faultily deciding the difference between the magnitude of the current I1 and the magnitude of the current I2 as an electrical leakage, the leakage detection circuit 13 needs to be set so that it does not decide an occurrence of electrical leakage even if a small unbalanced current is detected through the zero-phase-sequence current transformer ZCT. Therefore, this conventional example cannot detect such an electrical leakage whose magnitude is less or comparable with the magnitude of the current I3. That is, this conventional example cannot accurately detect occurrence of electrical leakage.